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<h1>Extending GoJS</h1>
<p>
  <b>GoJS</b> can be extended in a variety of ways.
  The most common way to change the standard behavior is to set properties on the <a>GraphObject</a>, <a>Diagram</a>, <a>CommandHandler</a>, <a>Tool</a>, or <a>Layout</a>.
  But when no such properties exist, you might need to override methods of CommandHandler, Tool, Layout, Link, or Node.
  Methods that you can override are documented in the API reference.
  This page describes how to override methods, either by replacing a method on an instance (a feature of JavaScript) or by defining a subclass.
  You should not modify the prototypes of any of the <b>GoJS</b> classes.
</p>

<p class="box bg-danger">
  Do not modify the prototypes of the <b>GoJS</b> classes.<br />
  Only use the properties and methods documented in the <a href="../api/index.html">API</a>.
</p>

<p class="box bg-info">
  In addition to our samples, <b>GoJS</b> provides an <strong><a href="../extensions/">extensions gallery</a></strong>,
  showcasing the creation of custom tools and layouts.
  Those classes and samples have been translated into TypeScript, available at <code>../extensionsTS/</code>.
  Those extension classes are UMD modules; they use the <code>../release/go.js</code> library.
  The extension classes are also available as ES6 modules at <code>../extensionsJSM/</code>; these use the <code>../release/go-module.js</code> library.
  We recommend that you copy the files that you need into your project, so that you can adjust how they refer to the "go.js" library
  and so that you can include them into your own building and packaging procedures.
</p>

<p class="box bg-danger">
  Note that the API for extension classes may change with any version, even point releases.
  If you intend to use an extension in production, you should copy the code to your own source directory.
</p>

<p class="box bg-danger">
  When using modern JavaScript, use arrow functions (<code>e => {}</code>) for event handlers and conversion functions.
  Use <code>function</code> only when overriding a method and you want access to <code>this</code> object
  whose method you have overridden.
</p>

<h2 id="CommandHandler">Command Handler</h2>
<p>
  Overriding the <a>CommandHandler</a> allows you to alter default functionality and create your own key bindings.
  See the <a href="commands.html">intro page on Commands</a> for more.
  However, the techniques shown below for overriding methods on Tools and Layouts also applies to the CommandHandler.
</p>

<h2 id="ToolsAndLayouts">Tools and Layouts</h2>
<p>
  <b>GoJS</b> operates on nodes and links using many tools and layouts, all of which are subclasses of the <a>Tool</a> and <a>Layout</a> classes.
  See the <a href="tools.html">intro page on Tools</a> for more about Tools, and the <a href="layouts.html">intro page on Layouts</a> for more about Layouts.
</p>
<p>
  Tools can be modified, or they can be replaced in or added to the <a>Diagram.toolManager</a>.
  All tools must inherit from the <a>Tool</a> class or from a class that inherits from Tool.
</p>

<p class="box bg-info">
  Some of our samples, such as the <a href="../samples/pipes.html">Pipes sample</a>, contain examples of custom tools.
  More custom tool examples are available in the <a href="../extensions/">extensions gallery</a>.
  TypeScript versions of those classes and samples are available in <code>../extensionsTS/</code>.
  Those custom tool classes are also available as ES6 modules in <code>../extensionsJSM/</code>.
</p>

<p>
  Layouts can be modified, or they can be used by setting <a>Diagram.layout</a> or <a>Group.layout</a>.
  All Layouts must inherit from the <a>Layout</a> class or a class that inherits from Layout.
</p>

<p class="box bg-info">
  Some of our samples, such as the <a href="../samples/parseTree.html">Parse Tree sample</a>, contain examples of custom layouts.
  More custom layout examples are available in the <a href="../extensions/">extensions gallery</a>.
  TypeScript versions of those classes are available in <code>../extensionsTS/</code>.
  Those custom layout classes are also available as ES6 modules in <code>../extensionsJSM/</code>.
</p>

<h2 id="OverridingMethodWithoutDefiningSubclass">Overriding a Method Without Defining a Subclass</h2>
<p>
  Often we can avoid subclassing a Tool in its entirety and merely override a single method.
  This is common when we want to make a small change to the behavior of a method.
  Here we show how to override the <code>ClickSelectingTool.standardMouseSelect</code> method by modifying the tool instance of a particular Diagram.
</p>

<p>
  One can override Layout methods in this manner also, but that is rarely done -- layouts are almost always subclassed.
  It cannot be done for layouts that are the value of <a>Group.layout</a> because those layouts may be copied and cannot be shared.
</p>

<p>
  Since we are not creating a new (sub)class, we set the method directly on the Diagram's <a>ClickSelectingTool</a>, which is referenced through its <a>ToolManager</a>.
  Typical scaffolding for overriding a method in such a manner is as follows:
</p>

<pre class="lang-js"><code>
  var tool = diagram.toolManager.clickSelectingTool;
  tool.standardMouseSelect = function() {
    // Maybe do something else before
    // ...

    // Be careful about using 'this' within such functions!

    // In cases where you want normal behavior, call the base functionality.
    // Note the reference to the prototype
    // and the call to 'call' passing it what 'this' should be.
    go.ClickSelectingTool.prototype.standardMouseSelect.call(tool);

    // Maybe do something else after
    // ...
  }
</code></pre>

<p>
  As a concrete example, we will override <a>Tool.standardMouseSelect</a> to select only Nodes and Links that have a width and height of less than 50 diagram units.
  This means that we must find the to-be-selected object using <code>diagram.findPartAt</code>, check its bounds, and quit if the bounds are too large.
  Otherwise, we call the base functionality to select as we normally might.
</p>

<pre class="lang-js" id="tool"><code>
  diagram.nodeTemplate =
    $(go.Node, "Auto",
      $(go.Shape, "Rectangle",
        { fill: "white" },
        new go.Binding("fill", "color")),
      $(go.TextBlock,
        { margin: 5 },
        new go.Binding("text", "key"))
    );

  var tool = diagram.toolManager.clickSelectingTool;
  tool.standardMouseSelect = function() {
    var diagram = tool.diagram;
    var e = diagram.lastInput;
    // to select containing Group if Part.canSelect() is false
    var curobj = diagram.findPartAt(e.documentPoint, false);
    if (curobj !== null) {
      var bounds = curobj.actualBounds;
      // End the selection process if the bounds are greater than 50 width or height
      if (bounds.width > 50 || bounds.height > 50) {
        // If this was a left click with no modifier, we want to act the same as if
        // we are clicking on the Diagram background, so we clear the selection
        if (e.left && !e.control && !e.shift) {
          diagram.clearSelection();
        }
        // Then return, so that we do not call the base functionality
        return;
      }
    }
    // otherwise, call the base functionality
    go.ClickSelectingTool.prototype.standardMouseSelect.call(tool);
  }

  diagram.model = new go.Model([
    { key: "Alpha", color: "lightblue" },
    { key: "Epsilon", color: "thistle" },
    { key: "Psi", color: "lightcoral" },
    { key: "Gamma", color: "lightgreen" }
  ]);
</code></pre>
<p>
  Running this code, we see that the "Epsilon" and "Gamma" nodes are not selectable, because they are both wider than 50.
  Note that this custom tool does not change the behavior of other tools that might select, such as the <a>DraggingTool</a> or the <a>DragSelectingTool</a>.
</p>
<script>goCode("tool", 300, 130)</script>


<h2 id="OverridingMethodsBySubclassingLayout">Overriding Methods by Subclassing a Layout</h2>
<p>
  Layouts can be subclassed to create custom Layouts that inherit the properties and methods of an existing layout.
  Subclassing in traditional JavaScript for <b>GoJS</b> requires a few key steps:
</p>

<ul>
  <li>Create a new class (function), and call the base class constructor.
  <li>Call <a>Diagram,inherit</a> with the new class and the base class.
  <li>Modify the prototype of your derived class to add new functionality.
</ul>

<p>
  To create a new Layout, called <code>CascadeLayout</code>, we would start with the following scaffolding:
</p>
<pre class="lang-js"><code>
  function CascadeLayout() {
    // Note the direct constructor call, no use of "prototype"
    go.Layout.call(this);
    // new properties go here, on "this"
  }
  go.Diagram.inherit(CascadeLayout, go.Layout);

  // Note setting the method on the prototype
  CascadeLayout.prototype.doLayout = function(coll) {
    // Layout logic goes here.
    // You can reliably use "this" to refer to the layout instance
    // on which this method was called.
  }
</code></pre>

<p>
  Note that if you are writing in modern JavaScript (ECMAScript 6) or in TypeScript, you can use newer syntax for defining classes:
</p>
<pre class="lang-js"><code>
  export class CascadeLayout extends go.Layout {
    // new data properties (fields) get declared and initialized here

    constructor() {
      super();
      // other initializations can be done here
    }

    // optionally, define property getters and setters here

    // override or define methods
    public doLayout(coll) {
      // Layout logic goes here.
    }
  }
</code></pre>

<p>
  Layouts commonly need additional properties that act as layout options.
  To add a "offset" property to <code>CascadeLayout</code>, we will use the convention that an underscore member is private, and will set a default value in the constructor:
</p>
<pre class="lang-js"><code>
  function CascadeLayout() {
    go.Layout.call(this);
    this._offset = new go.Size(12, 12);
  }
</code></pre>

<p>
  Then, we use <code>Object.defineProperty</code> to make a "public" getter and setter.
  Getters and setters allow us to do type checking and have side effects.
  This setter makes sure the offset value is a <code>go.Size</code> object and invalidates the layout only if the value has changed.
</p>
<pre class="lang-js"><code>
  Object.defineProperty(CascadeLayout.prototype, "offset", {
    get: function() { return this._offset; },
    set: function(val) {
      if (!(val instanceof go.Size)) {
        throw new Error("new value for CascadeLayout.offset must be a Size, not: " + val);
      }
      if (!this._offset.equals(val)) {
        this._offset = val;
        this.invalidateLayout();
      }
    }
  });
</code></pre>

<p>
  If you are writing in ECMAScript 6 or TypeScript, you can define property getters and setters.
</p>
<pre class="lang-js"><code>
  get offset() { return this._offset; }
  set offset(val) {
    if (!(val instanceof go.Size)) {
      throw new Error("new value for CascadeLayout.offset must be a Size, not: " + val);
    }
    if (!this._offset.equals(val)) {
      this._offset = val;
      this.invalidateLayout();
    }
  }
</code></pre>

<p>
  If the layout might be used as the value of <a>Group.layout</a>,
  you will need to make sure the instance that you set in the Group template can be copied correctly.
</p>
<pre class="lang-js"><code>
  CascadeLayout.prototype.cloneProtected = function(copy) {
    go.Layout.prototype.cloneProtected.call(this, copy);
    copy._offset = this._offset;
  }
</code></pre>

<p>
  Lastly we'll define a <a>Layout.doLayout</a>, being sure to look at the documentation and accomodate all possible input, as doLayout has one argument that can either be a <a>Diagram</a>, or a <a>Group</a>, or an <a>Iterable</a> collection.
</p>
<p>
  All together, we can see the cascade layout in action:
</p>
<pre class="lang-js" id="example"><code>
  /**
  * @constructor
  * @extends Layout
  * @class
  * This layout arranges nodes in a cascade specified by the offset property
  */
  function CascadeLayout() {
    go.Layout.call(this);
    this._offset = new go.Size(12, 12);
  }
  go.Diagram.inherit(CascadeLayout, go.Layout);

  CascadeLayout.prototype.cloneProtected = function(copy) {
    go.Layout.prototype.cloneProtected.call(this, copy);
    copy._offset = this._offset;
  }

  Object.defineProperty(CascadeLayout.prototype, "offset", {
    get: function() { return this._offset; },
    set: function(val) {
      if (!(val instanceof go.Size)) {
        throw new Error("new value for CascadeLayout.offset must be a Size, not: " + val);
      }
      if (!this._offset.equals(val)) {
        this._offset = val;
        this.invalidateLayout();
      }
    }
  });

  /**
  * This method positions all Nodes and ignores all Links.
  * @this {CascadeLayout}
  * @param {Diagram|Group|Iterable} coll the collection of Parts to layout.
  */
  CascadeLayout.prototype.doLayout = function(coll) {
    // get the Nodes and Links to be laid out
    var parts = this.collectParts(coll);

    // Start the layout at the arrangement origin, a property inherited from Layout
    var x = this.arrangementOrigin.x;
    var y = this.arrangementOrigin.y;
    var offset = this.offset;

    var it = parts.iterator;
    while (it.next()) {
      var node = it.value;
      if (!(node instanceof go.Node)) continue;  // ignore Links
      node.move(new go.Point(x, y));
      x += offset.width;
      y += offset.height;
    }
  }
  // end of CascadeLayout

  // Regular diagram setup:

  diagram.layout = new CascadeLayout();

  diagram.nodeTemplate =
    $(go.Node, "Auto",
      $(go.Shape, "Rectangle",
        { fill: "white" },
        new go.Binding("fill", "color")),
      $(go.TextBlock,
        { margin: 5 },
        new go.Binding("text", "key"))
    );

  diagram.model = new go.Model([
    { key: "Alpha", color: "lightblue" },
    { key: "Beta", color: "thistle" },
    { key: "Delta", color: "lightcoral" },
    { key: "Gamma", color: "lightgreen" }
  ]);
</code></pre>
<script>goCode("example", 300, 200)</script>

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